The present invention relates to electrochemical cells and methods for assembling electrochemical cells.
Electrochemical cells utilizing a proton exchange membrane (PEM) can be configured in cells stacks having bipolar separator plates between adjacent cells. These bipolar separator plates are typically made from a variety of metals, such as titanium and stainless steel, and non-metallic conductors, such as graphitic carbon. Bipolar separator plates are typically fabricated by machining flow fields into a solid sheet of the material. Alternatively, when carbonaceous conductive materials are used, the precursor material is formed by injection molding and converted to the conductive carbon form by high temperature firing under carefully controlled conditions. The flow fields are made up of a series of channels or grooves that allow passage of gases and liquids.
FIG. 1 is a face view of a prior art bipolar separator plate 10 made from a solid sheet of a conducting material. The central portion of the plate has a flow field 12 machined into its surface. The flow field may direct fluid flow across the surface of an electrode in many patterns, but is illustrated here as parallel serpentine channels. Around the perimeter of the flow field 12, the plate provides a plurality of bolt holes 14 for assembling and securing a cell stack, various manifolds 16 for communicating fluids through the stack, and a flat surface 18 that allows the plate to be sealed against adjacent components of the cell stack.
In addition to providing a fluid flowfield, a bipolar separator plate for use in electrochemical cells must collect electrons liberated at one electrode (i.e., an anode), conduct the electrons through the plate, and deliver electrons to the face of another electrode (i.e., a cathode) on the opposing side of the plate. The bipolar plate shown in FIG. 1, collects and delivers electrons from electrodes of opposing cells through contact between the electrodes and the ridges 20 remaining between the channels 22 in the flowfield 12.
FIG. 2 is a schematic view of a proton exchange membrane (PEM) electrochemical cell configured as a hydrogen-air fuel cell stack 30. This stack 30 comprises two identical fuel cells 32 each having a cathode 34, a PEM 36 and an anode 38. Flow fields 40 (shown schematically for clarity) are provided on either side of the bipolar separator plate 42, as well as on the internal faces of the endplates 44. Electrons liberated at the anodes 38 induce electronic current flow to the cathode 34 of an adjacent cell on the other side of the plate 42 and, in the case of the last anode of the stack (here the anode on the right of the page), through an external circuit 46. Electrons are then combined with protons and oxygen at the cathodes 34 to form water. The electrical potential of the fuel cell 30 is increased by adding more cells 32 to the stack.
While the foregoing methods are relatively straight forward, they have several disadvantages. First, the solid piece of graphite or metal used to fabricate the bipolar separator plate constrains the density of the final product to a density approximately the same as that of the original stock, thereby producing a very dense and heavy bipolar separator plate. Second, machining each piece from a solid starting blank requires relatively expensive machining processes, as opposed to less expensive molding, casting or stamping processes. When carbon components are used the molding step is inexpensive, however, the controlled sintering required to convert the precursor to the final product is slow and requires precise atmosphere and temperature control throughout the process.
Another important aspect in fabricating an electrochemical fuel cell is the number of joints and junctions created in the cell. Reduction of the number of joints and junctions can greatly improve the performance of a electrochemical cell stack, for example if fabricated from a stack of flat components, because there are fewer potential leak points and fewer electronic contact resistances. A fabrication process that provides an electrochemical cell with a minimum of joints and junctions would be highly desirable.
Assembling a PEM fuel cell stack using relatively flat components requires gas tight seals at each interface. Gaskets are typically used to create gas tight seals, however gaskets increase the number of parts that must be fabricated and aligned when the stack is assembled. A method and apparatus for forming gas tight bonds or seals at the interfaces between the components of an electrochemical cell would obviate the need for several gaskets and produce a more efficient cell.
Therefore, there remains a need for an improved bipolar separator plate. It would be desirable if the bipolar separator plate were thin, light weight, and could support high current densities. It would be further desirable if the bipolar separator plate reduced the number of joints or junctions in the individual cells or a cell stack and reduced the need for gaskets. Furthermore, it would be useful if the structure of the bipolar separator plate allowed the introduction of other specific properties, such as water permeability and reactant gas impermeability.
The present invention provides a method for preparing a subassembly for an electrochemical cell. The method includes aligning a subassembly having two or more electrochemical cell components with one or more bonding elements disposed between the two or more electrochemical cell components. The bonding elements have a melting point temperature that is lower than the melting point temperature of any one of the two or more electrochemical cell components. The subassembly is compressed and heated to a temperature that is between about the melting point temperature of the bonding element and about the lowest melting point temperature of the any one of the two or more electrochemical cell components. Preferably the temperature is less than 800xc2x0 C., more preferably below 250xc2x0 C. The subassembly is then allowed to cool.
The subassembly is preferably positioned into an electrochemical cell or an electrochemical cell stack. The two or more electrochemical cell components are preferably metal components selected from plates, shims, frames, flow fields or combinations thereof, such as stainless steel, titanium, nickel, nickel plated aluminum, nickel plated magnesium, or combinations thereof. The bonding element is preferably solder. The metal component is preferably dipped in a flux; then dipped in a bonding metal or solder, such as tin or a silver-tin alloy. The bonding metal, or solder can also be applied to the metal surface by electrodeposition or by various vacuum deposition techniques.
Light or easily oxidized metal components, such as those made from aluminum, magnesium, or alloys containing aluminum or magnesium are preferably coated with a layer of a corrosion resistant transition metal prior to the dipping the metal component in the flux. Suitable corrosion resistant transition metals include but are not limited to cobalt, copper, silver, nickel, gold or combinations thereof. Nickel is the most commonly used metal for the corrosion resistant layer.
Alternatively, the two or more electrochemical cell components can be polymer components selected from frames, gaskets, membranes, shims, or combinations thereof where the bonding element is preferably an adhesive. The two or more electrochemical cell components may also comprise one or more metal components and one or more polymer components, where the bonding element is an adhesive.
The two or more electrochemical cell components can include a plate and a flow field. The subassembly preferably includes a bipolar plate and a frame. The bipolar plate preferably has two plates, a flow field and a frame. The frame and flow field are disposed between the two plates with the frame disposed around the flow field. The frame has channels in fluid communication with the flow field.
In another embodiment of the invention, there is provided a fluid cooled bipolar plate assembly having two electronically conducting plates having opposing faces, an electronically conducting flow field bonded in electronic communication with a substantial portion of the opposing faces of the plates, between the two electronically conducting plates, and a frame disposed around a perimeter of the electronically conducting flow field and bonded between the two electronically conducting plates. The frame has channels for providing fluid communication between the flow field and a fluid source.
Preferably, an electronically conducting cathode flow field and an electronically conducting anode flow field are bonded to opposing sides of the assembly. The assembly is preferably bonded with an adhesive and/or solder.
In yet another embodiment there is provided a bipolar plate for electrochemical cells having two or more porous, electrically conducting sheets selected from expanded metal mesh, woven metal mesh, metal foam, conducting polymer foam, porous conductive carbon material or combinations thereof. An electrically conducting gas barrier is disposed in electrical contact between the sheets. A cell frame is disposed around a periphery of any one of the two or more porous electrically conducting sheets. The cell frame has at least one surface that is bonded to the gas barrier.
Preferably, the cell frame includes channels in fluid communication with the porous electrically conducting sheet. The cell frame can be metallic, where it is bonded to the gas barrier with a metallic bond. The metallic bond is preferably formed by soldering the cell frame to the gas barrier. Alternatively, the cell frame can be polymeric, where it is bonded to the gas barrier with a polymeric bond. Preferably the polymeric bond is produced by an adhesive.